Information displays for instruments, televisions, computers and the like are widely used. In an effort gain display ample, correct, concise, and high-speed information on a machine, the display elements of these information displays have been widely studied.
While cathode ray tubes (CRTs) clearly dominate the market with their bright, saturated colours, they are also known to be heavy, power consuming and bulky. For these reasons, flat panel displays are a highly attractive alternative for computers, television, wall-mounted large-screen video displays and a wide range of other applications.
An example of a flat panel display is an active-matrix liquid-crystal display, these displays being commercially available. Even though this technology is now widely used for laptop computer displays, in general it is not considered to be a widespread replacement for the CRT technology. The major shortcomings of the LCD-based display are that it is an inefficient colour subtractive technology, requiring a power consumptive backlight. Also, it is relatively slow, and has a narrow viewing angle. One alternative to LCDs is based on conventional semiconductor light-emitting diode (LED) technology. However, very high costs associated with the requirement of epitaxial multilayer structures make them an unlikely choice for use in low-cost displays in the near future.
A promising flat panel display free from the above-mentioned disadvantages is based on organic light-emitting diodes (OLEDs) which use an organic luminescent material for light emission. The organic luminescent materials are very attractive due to their versatility, richness in blue photoluminescence, and high photo-luminescent quantum yields.
Further advantages of the OLED display are that they are self-luminous, light weight, capable of high-speed response, and independent of viewing angle. It is expected that these advantages will be successfully exploited, and that a commercial use for organic EL devices will be realised in the near future.
To obtain high-performance OLEDs with low carrier injection barriers, high electroluminescence (EL) efficiency and long lifetime, materials design and device configurations are two important factors. It is desirable that the OLED materials possess the following properties: good carrier transport properties, high photoluminescence (PL) quantum yield, and suitable ionisation potential (IP) and/or electron affinity (EA). The synthesis of highly fluorescent and stable materials that can be utilised in OLEDs is one of the most challenging works in this field.
Many materials are known which emit green or blue light. However, few satisfactory red-emitting materials are known.
Some organic compounds with red emission have been reported, such as pyran-containing compounds, porphyrin compounds and europium metal complexes. Due to the instability of rare earth metal complexes during thermal deposition, no europium metal complexes show a practical operation lifetime, even though they have a sharp emission peak with high colour purity. A known red dopant from the porphyrin group of compounds is platinum octaethylporphyrin (PtOEP), however devices using this compound doped in AlQ3 do not have brightness or chromaticity which is acceptable for practical applications. Pyran-containing compounds include 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), 4-(dicyanomethylene)-2-methyl-6-(1,1,7,7-tetramethyljulolidin-9-yl)-4H-pyran (DCJT) and 4-(dicyanomethylene-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-9-yl)-4H-pyran (DCJTB). A disadvantage of these compounds is that they have a highly concentration-dependent emission. A desirable red colour can only be obtained at high concentration, which dramatically reduces luminance efficiencies.
Thus, despite significant developments in the field of OLEDs, red emission from OLEDs with high colour purity and good stability is still needed.
Polycyclic aromatic hydrocarbons (PAHs) have very high PL quantum yield and have already been used in OLEDs. The pure hydrocarbon conjugated structure of the compounds intrinsically determines their relatively high carrier transport abilities. Most PAHs are highly luminescent and relatively stable. Examples using this kind of material in OLEDs include T. Sano et al, Syn. Met., 91, 27 (1997), and U.S. Pat. Nos. 5,935,721 and 5,858,564.
It is known that the energy gap of PAH compounds depends on the conjugation length in the molecules, with a longer conjugation length resulting in a smaller HOMO-LUMO energy gap. Materials with red emission generally have a narrow energy gap. However, many known PAHs emit only in the blue and yellow regions, with only a few PAH compounds reported to have emission about 600 nm. Moreover, PAH compounds with more fused carbon rings, and hence a longer conjugation length, have a high sublimation temperature, which limits the application of red-emission PAH compounds in OLEDs.